(1) Field of the Invention
The present invention relates to a process for preparing a ferrierite type zeolite. More particularly, it relates to a process for preparing a ferrierite type zeolite whereby a ferrierite type zeolite having a high purity and a high SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio is prepared easily with a good reproducibility without using an organic or inorganic mineralizing agent.
(2) Description of the Related Art
Zeolite is a crystalline aluminosilicate containing zeolitic water, as is apparent from the fact that the origin of the word "zeolite" is a Greek word "Zein" which means "boiling stone", and zeolite is generally represented by the following empirical formula: EQU xM.sub.2/n O.Al.sub.2 O.sub.3.ySiO.sub.2.zH.sub.2 O
wherein n stands for the valency of the cation M, x is a number of from 0.8 to 2, y is a number of at least 2, and z is a number of at least 0.
The basic structure of the zeolite comprises SiO.sub.4 tetrahedrons containing silicon at the center and four oxygen atoms at the apexes and AlO.sub.4 tetrahedrons containing aluminum at the center instead of silicon, in which the tetrahedrons are regularly three-dimensionally bonded together while owning oxygen atoms in common, so that the O/(Al+Si) atomic ratio is 2. Accordingly, there is formed a three-dimensional network structure having pores differing in the size and shape according to the difference of the manner of bonding of tetrahedrons.
The negative charges of the AlO.sub.4 tetrahedrons are electrically neutralized by coupling with cations such as alkali metal or alkaline earth metal ions.
The thus-formed fine pores generally have a size from 2 to 3 angstroms to ten-odd angstroms and the size of the fine pores can be changed by exchanging metal cations bonded to the AlO.sub.4 tetrahedrons with other metal cations having a size or valency different from that of the bonded metal cations.
By utilizing these fine pores, the zeolite is used as an industrial dehydrator for a gas or liquid or as a molecular sieve for adsorbing and separating specific molecules from mixtures containing at least two kinds of molecules. Furthermore a product obtained by exchanging the metal cations with hydrogen ions acts as a solid acid, and is used as an industrial catalyst for various purposes by utilizing the property as the solid catalyst.
A ferrierite type zeolite is naturally produced, and its typical composition is represented as follows: EQU (Na.sub.2,Mg)O.Al.sub.2 O.sub.3.11.1SiO.sub.2.6.5H.sub.2 O
The crystal structure of the zeolite comprises 5-membered oxygen ring units constituting the skeleton, and is characterized in that it has fine pores of 10-membered oxygen rings having a size of 4.3.times.5.5 angstroms and fine pores of 8-membered oxygen rings having a size of 3.4.times.4.8 angstroms.
Various processes as described below have been proposed for synthesizing ferrierite type zeolites. However, an industrially satisfactory process has not been developed yet.
(1) In the process proposed by C. L. Kibby et al (Journal of Catalysts, Vol. 35, pages 256-272, 1974), a high temperature exceeding about 300.degree. C. is necessary for crystallization, and therefore, a high-temperature high-pressure type reaction vessel must inevitably be used.
(2) In the process disclosed in Japanese Unexamined Patent Publication No. 51-106700, the synthesis can be carried out at a relatively low temperature, but a starting silica-alumina material having a specific composition should be used and the presence of a potassium ion in the reaction system is indispensable. Moreover, addition of a mineralizing agent in the form of a potassium salt of a specific organic or inorganic polybasic acid is indispensable. Thus, in this process, selection of the starting materials and setting of the reaction conditions are complicated.
(3) In the process disclosed in Japanese Unexamined Patent Publications Nos. 50-127898 and No. 55-85415, use of N-methylpyridine hydroxide and piperidine and/or an alkyl-substituted piperidine as an organic mineralizing agent is indispensable. These organic amines are expensive, and since these organic amines are included in the formed zeolite, when the ferrierite type zeolite obtained according to this process is used as an adsorbent or catalyst, it is necessary to calcine the once-synthesized zeolite at a high temperature exceeding 500.degree. C. in the presence of oxygen to remove the amines. Namely, when the zeolite is used for these specific purposes, the zeolite should be subjected to a preliminary treatment as described above.
(4) The process disclosed in Japanese Unexamined Patent Publication No. 53-144500 is a process for synthesizing a ferrierite type zeolite called "ZSM-35", in which butane diamine or an organic base-containing cation derived therefrom is used. As in the above-mentioned process (3), the zeolite obtained by the synthesis should be subjected to a preliminary treatment before it is used in various fields.
It is known that in zeolites comprising 5-membered oxygen rings as the zeolite skeleton-constituting units, such as ferrierite, mordenite and ZSM-5, the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio can be increased to a relatively high level. However, for the synthesis of these zeolites, it is ordinarily indispensable that an organic nitrogen-containing compound or other organic compounds should be added to the reaction system, as pointed out hereinbefore. Furthermore, in these known processes, in order to enhance the activity of the reaction mixture, expensive aqueous colloidal silica is ordinarily used as the silica source.